For a wide variety of technological processes, it is necessary to determine the grain-size distribution or particle size distribution of powders, e.g. a particulate having a grain size between 0.1 micrometer and 200 micrometers. One possible approach to such distribution measurements is to monitor the sedimentation process of the powder in a liquid since larger particles sediment more rapidly than smaller particles. This can be done, for example, by the method according to Andreas as set out in German Industrial Standards DIN 66 111 and DIN 66 115.
In this system, samples are taken at time intervals from the region of the bottom of the vessel in which the particles are caused to sediment from the liquid. The solids content of the particles can be used as a measurement of the particle diameter or the sieve size through which the particles could be expected to pass as calculated utilizing the additional parameters of particle density, liquid density, viscosity and fall height, and of course, time. A sedimentation balance operates under similar principles except that here the sedimentation particles are not drawn off by suction but are collected on the balance. The increase in weight with respect to time permits calculation of the grain size distribution.
The sedimentation process can be monitored, however, in another way. For example, it can be followed by an arrangement which includes a horizontal light beam or ray located in the vicinity of the bottom of the vessel above the body forming the bottom. With this arrangement, a measurement of the attenuation of the light beam by the suspended particles can be made. Initially the light attenuation by the suspended particles is comparatively great and after sedimentation of the larger particles, more light is permitted to traverse the cuvette to reach the sensor responding to the light transmission through the cuvette. From the change in attenuation with time, the particle size distribution can be calculated. An electrical light sedimentometer using these principles is described by O. Telle in V.D.I. Berichte, volume 7, 1955.
This system has, however, a significant disadvantage since the measurement takes a long time. If one wishes to ascertain with reasonable precision the large particle fraction of a suspension containing a wide variety size range, the cuvette must be so high that the larger particles or grains take from 5 to 20 seconds and even up to 30 seconds to sediment out, (see T. Allen, Particle Size Measurement, Chapman and Hall, London). If a shorter time span is used, this approach becomes inaccurate.
A tall cuvette of the time required for such fall times means that the fall times for the smallest particles can be up to 12 to 14 hours, making the entire process prohibitively long. To eliminate this disadvantage of an excessively long sedimentation time, devices have come on the market in which the measuring cuvette with the suspension is moved vertically from above downwardly relative to the light beam.
Hence the light beam initially traverses the cuvette in the region of the bottom thereof and toward the end of the process traverses the light beam in the vicinity of the upper surface. In this case, one need not await the migration of the smallest particles to the bottom of the cuvette.
The measuring process, depending upon the desired measurement particle size range, can be shortened to 15 to 30 minutes.
The reverse principles of operation can, of course, also be used, i.e. the light beam or, more specifically, the light "curtain" defined by the light source on one side of the cuvette, the light detector or sensor on the opposite side of the cuvette, and the ray passing between the source or emitter and the detector or sensor, can be moved along the cuvette upwardly.
Both of the principles just described do indeed avoid the drawback of prolonged measuring times, although they have significant disadvantages. As a result of the movement of the cuvette wall relative to the light beam, any optical characteristics of the wall that may differ from place to place therealong may introduce inaccuracies. These deviations in the optical properties of the wall can include wall thickness, refraction angle, parallelity of the wall surface of transparency. Additionally, relative movement causes wear which can contribute to inaccuracy and finally any actual movement of the cuvette can disturb the sedimentation process.